The overarching goal of this project is to study social engagement in infants with autism spectrum disorders (ASD) by means of eye-tracking technology and innovative quantification of visual attention during viewing of naturalistic social situations. We will measure (1) visual fixation time during viewing of adults engaged in infant-directed approaches, and (2) temporally-sensitive visual scanning patterns during viewing of infants engaged in peer play. A cohort of 200 12- to 24-month-old infants will be shared with Projects 2 and 3, consisting of infants with ASD (N=70), non-autistic developmental delays (DD) (N=70) and typical development (TD) (N=60). The ASD and DD cohorts will be re-evaluated at age 36 to 48 months for confirmatory diagnosis and measures of developmental outcome; the TD group will be screened for false negatives at that time. This work builds on our findings of anomalous visual fixation patterns to dynamic social stimuli in adolescents (R01 HD04217) and in 24- to 36-month-olds (U54 MH66494, Yale STAART) with ASD. In both cases, summaries of visual fixation on regions of interest (eyes, mouth, body, object) were strong predictors of concurrent, standardized measures of social disability. In this project, we propose to downward extend this research to the second year of life, which is currently the period of earliest detectability of ASD. We also propose to employ novel group measures of moment-by-moment visual scanning behavior developed by our group, which are particularly sensitive to time-delimited social and physical cues occurring naturallly in infants' surrounding environment. The project builds thematically and methodologically on various core goals of the Yale ACE application: (1) the study of early-emerging mechanisms of socialization that are potential mediating phenotypes of social and communicative functioning; (2) the assessment of their predictive power relative to developmental and diagnostic outcome; and (3) the development of performance-based screening protocols for infants at risk for ASD. More broadly, we will explore the role of these mechanisms on heterogeneity of syndrome manifestation. This project addresses several key action items of the NIH Interagency Autism Coordinating Committee, emphasizing developmental markers and screening in infants, and neurodevelopmental processes.